1. Field of the Invention
This invention relates generally to electrochemical cells. In particular, this invention relates to an improved frame for proton exchange membrane electrochemical cells that facilitates the movement of fluids and gases through the cell, thereby facilitating operation at high pressure and high fluid flow rates.
2. Brief Description of the Related Art
Electrochemical cells are energy conversion devices, usually classified as either electrolysis cells, fuel cells or batteries. The typical proton exchange membrane electrochemical cell stack includes a number of individual cells arranged in a stack with fluid, typically water flowing therein. The fluid is typically forced through the cells at high pressures. The cells within the stack are sequentially arranged including an anode, a proton exchange membrane, and a cathode. The anode/membrane/cathode assemblies are supported on either side by layers of screen or expanded metal which are in turn surrounded by cell frames and separator plates to form reaction chambers and to seal fluids therein. The cell frames are typically held together in the stack by tie rods passing through the frames and separator plates. End plates are mounted to the outside of the stack and together with the cell frames and tie rods function to react the pressure of the fluids operating within the stack. The frames include ports to communicate fluids from a source to the individual cells and also include additional ports to remove the fluids from the cells. The screens are used to establish flow fields within the reaction chambers to facilitate fluid transport, to maintain membrane hydration, provide mechanical support for the membrane, and provide a means of transferring electrons to and from electrodes.
A proton exchange membrane electrolysis cell, for example, functions as a hydrogen generator by electrolytically decomposing water to produce hydrogen and oxygen gas. Referring to FIG. 1, in a typical prior art single anode feed water electrolysis cell 101, process water 102 is reacted at oxygen electrode (anode) 103 to form oxygen gas 104, electrons, and hydrogen ions or protons 105. A portion of the process water containing some dissolved oxygen gas 102' and oxygen gas 104, exit the cell. The protons 105 migrate across a proton exchange membrane 108 to a hydrogen electrode (cathode) 107 where the protons 105 react with the electrons to form hydrogen gas 109. The hydrogen gas 109 and water 102" drawn across the membrane 108 by the protons (hydronium ions), exit from the cell through manifolds in the cell stack. Reactions for a typical electrolysis cell are as follows:
Anode: 2H.sub.2 O.fwdarw.4H.sup.+ +4e.sup.- +O.sub.2 PA1 Cathode: 4H.sup.+ +4e.sup.- .fwdarw.2H.sub.2 PA1 Anode: 2H.sub.2 .fwdarw.4H.sup.+ +4e.sup.- PA1 Cathode: 4H.sup.+ +4e.sup.- +O.sub.2 .fwdarw.2H.sub.2 O
A typical fuel cell operates in the reverse manner as that described herein above for electrolysis cells. In a fuel cell, hydrogen, methanol, or other hydrogen fuel sources combine with oxygen, via the assistance of a proton exchange membrane, to produce electric power. Reactions for a typical fuel cell are as follows:
The cell frames which surround each of the cells within a stack typically contain multiple ports for the passage of reactant fluids. These ports are usually sealed by means of sealing ridges which are embossed, machined, or molded into the frame. The sealing features react against gaskets included in the stack to maintain fluid tight joints and also grip the gaskets to prevent creep and extrusion of the membrane. As is well known in the art and discussed in U.S. Pat. No. 5,441,621 to Molter et al., a common method of sealing utilizes ridges in concentric patterns around the ports and separate concentric patterns around the sealing area of the cell frame.
A common method for providing a fluid communication pathway between the reservoir, or active area, of a cell and individual fluid ports in the frame comprises manifolds machined into the frame. The manifolds typically comprise holes machined in the edge of the cell frames orthogonal to the ports. Fluids, after passing through the inlet ports and the holes in the manifolds, enter the screen packs, electrodes and membrane. The fluids and gas products similarly exit through outlet manifolds, sealed outlet ports, to collection tanks.
Existing cell frames have a number of drawbacks and disadvantages. For example, current technology uses protector rings to bridge the gap between the cell frame and screen packs. The protector rings, typically positioned about the perimeter of the frame, function to prevent membrane extrusion and "pinching" between the frame and the screen. Although these protector rings function well in operation, they render assembly of the cell very difficult, often breaking loose resulting in misalignment and possible damage to the membrane. Specifically, because of their small cross-section, the protector rings tend to slide out of position and as a result often do not cover the gap between the frame and the screen which they are intended to bridge.
Manufacture of conventional cell frames typically comprises injection molding of polymeric material. Although cell frames manufactured from polymeric materials function well in fuel cells, the fluid manifold details are difficult to machine. These details require drilling long, thin flow passages through the cell frame material. The drilling operations are expensive and usually very slow in order to prevent overheating and melting of the material. Fixturing the frame is difficult, and the process does not lend itself to any type of automation. FIG. 2 illustrates a typical fluid flow field 110 within a prior art cell frame 111. The flow field 110 is designed such that fluid 112 enters the cell through entrance manifold 113 located directly opposite outlet manifold 114 where fluid and products 115 exit the cell. This design, in conjunction with active area 116, of the typical cell, creates stagnant sections 117; sections of the cell where inadequate fluid flow occurs. Stagnant areas can cause poor fluid flux, membrane dehydration and overheating of that segment of the cell active area, which can result in cell failure and/or reduced cell life.
Different types of cells require different amounts of fluid flow. For instance, some cells, such as electrolysis cells which operate at high current density, require high flow rates of water to effect proper cell cooling, thereby necessitating a great number of manifolds with large flow areas to accommodate the fluid volume with minimal pressure drop. By contrast, in fuel cells, gaseous hydrogen and oxygen are supplied to the cell at stoichiometric rates, thereby requiring less manifold area in the cell frame. In addition, manifolds of differing capacity and position are frequently required for testing and for design capacity changes. Existing designs for electrochemical cells include large manifolds for flexibility in carrying these fluids. In order to maintain proper structural soundness in the sealing area, use of large manifolds requires large frame areas. This, in turn, requires that the application of a large axial load to the cell components in order to maintain component sealing, thereby significantly increasing the required end plate thickness and the size/strength of cell tie rods in order to minimize deflection under load.
Accordingly, there remains a need for cell frames which have leak-proof seals between the frame and the flow field, are less expensive to manufacture, provide optimal flow without stagnant areas, provide for flexibility in manifold size and positioning, and allow for high flow rates without the disadvantages attendant with using very large cell frames.